Refine and integrate the "propagator-AS" search functionality in the API

This is the functionality developed by Pei-Yi to search for events where a specific AS has contributed to the propagation of a potentially-malicious route announcement.

Start by locating the existing propagator-AS search functionality and the API entry points that should expose it. Read the related implementation and request handling flow, then determine the integration gaps. Done means the API supports searching for events where a specified AS contributed to propagating a potentially malicious route announcement.

Written by the indexing model from the issue text.